The features sizes of transistors used in integrated circuits have become significantly smaller as fabrication technology has advanced. For example, transistor feature sizes have scaled downward to the point that the oxide layer in a metal-on-oxide transistor is less than 10 angstroms thick. “Aging” effects that can result in instability of circuit components such as transistors have become more pronounced as the feature sizes have scaled downward. For example, in transistors fabricated with this thin oxide, phenomena have been observed whereby the threshold voltage (the voltage it takes to turn the transistor “on”) varies over time.
Many integrated circuits, such as microprocessors, employ thermal sensors which rely on precise calibration to provide temperature-related compensation in the performance of various circuit elements. A shift in the threshold voltage can alter the speed of an on-die thermal oscillator by 10 to15%. This will result in a skew of the calibration curve over the lifetime of the part, which is undesirable given the projected lifetime of 5 to 10 years for many circuit components.
One of the circuit elements that is particularly susceptible to aging effects is the phase-locked loop. Components used in the phase-locked loop can be affected by negative bias threshold instability (NBTI) that can alter the frequency range of a voltage controlled oscillator by 10% or more over a 10 year lifetime. This degradation is related to the drive current caused by increased PMOS strength. As a result, there is a need to alter other factors affecting drive current over time to compensate for the effects of NBTI and to ensure that the oscillator in the phase-locked loop achieves the same frequency range over its lifetime.
Another problem related to the aging effects of circuit components relates to operation of the delay-locked loop in a microprocessor. Similar to the effects on a phase-locked loop, NBTI effects can alter the lowest delay achieved by the delay-locked loop by 10% or more over the lifetime of the components. As a result, it is necessary to alter the factors affecting drive current such as the delay line in the delay-locked loop so that the delay-locked loop achieves the same frequency range over its entire lifetime.
In view of the foregoing, there is a need for a technique to compensate for aging effects in circuit components, such as transistors, in integrated circuits. Such a technique is provided in the various embodiments of the present invention, as discussed below.